Is it possible to minimize operating costs for industrial thermal processing equipment while maximizing productivity in combination drying and oxidizing systems? Yes, process optimization can be achieved via:
- A thorough understanding of the complicated relationship that exists between dryer systems and oxidation technologies.
- A comprehensive analysis of the needs of each variable component.
- The desire to improve overall process operations by adhering to a strict system design criteria.
Even when the drying processes are optimized and running smoothly, there is another dimension to achieving the perfect drying system design: the complicated relationship between the dryer and the emissions control device such as the oxidizer.
Minimizing the exhaust air from process dryers and oven systems is only a starting point for reducing oxidizer operating costs. An emphasis on air management within the process is critical to the design of pollution control systems for treating exhausts containing volatile organic compounds (VOCs). Efficient air management allows the pollution control system to be optimally sized to meet the needs of the process. Good process control further defines the emission stream by avoiding the need for large safety factors that increase control equipment size (and costs) and reduce efficiency.
To begin process optimization, key components need to be analyzed, including:
- Oxidizer efficiency.
- Control efficiency.
- Capture efficiency.
- Destruction efficiency.
- Oxidizer types.
- System design criteria.
- Airstream composition.
- Production schedule.
- Particulates and nonvolatile oxidation products.
- Exhaust rate and temperature.
- Self-sustaining operation.
- Secondary energy recovery options.
This article will look at each of these points.
Control Efficiency. Control efficiency for an emission control system is the product of two major items: capture efficiency and destruction efficiency. Capture efficiency includes the collection of VOCs at the dryer and the capture of fugitive emissions from the entire process system. Destruction efficiency is the level of removal provided once the VOCs are introduced into the emission control device.
Regulations demand high overall VOC control efficiencies. For example, many coating applications have overall emissions control requirements of 95 percent as standard. Additionally, certain geographic areas have regulations that require overall VOC control efficiencies of 98 percent.
Capture Efficiency. High capture efficiency of the VOCs is mandatory for most systems today. This may require enclosures around coating equipment. These coater “rooms” are known as permanent total enclosures (PTEs). They contain or capture 100 percent of the emitted process solvents, allowing them to be ducted directly to the pollution control system. The exhaust from these enclosures also is sometimes used as makeup air to the oven system. The decision as to how the PTE exhaust is handled is a function of process conditions and oven design.
Destruction Efficiency. Once the VOCs have been captured, they are sent to an emission control device. Oxidation systems are used as end-of-pipe pollution control solutions for emissions from many solvent-based processes. Along with VOCs, carbon monoxide, nitrogen oxide and other emissions must be processed to meet emissions requirements over a wider area.
The desired efficiency rate can vary depending upon process requirements. To ensure the most effective results are achieved, the oxidizer type should be matched to the specific needs of the application.
Oxidation systems are available in many designs and configurations. Various types of oxidizers, characterized by the operating temperatures, are used to control coating line VOC emissions. They include:
- Thermal: direct, recuperative, regenerative; 1400 to 1800°F (760 to 980°C).
- Catalytic: direct, recuperative, regenerative; 400 to 750°F (200 to 400°C).
Oxidizer design variations address differences in process conditions, operating schedules, installation space requirements and operating costs. Oxidizers are defined based upon the type of heat exchanger used and the presence (or absence) of a catalyst within the oxidizer airflow. It is important to remember that the end result expected from all of these systems is a 98 percent or higher destruction of the VOCs from the process.
Direct. Direct thermal oxidizers do not have heat exchangers. They are costly to operate. Normally, they are used only for extremely intermittent use such as small pilot operations.
Recuperative. Recuperative heat recovery systems typically use a metal shell-and-tube or plate-type heat exchanger to recover the energy used in the oxidation process.
Recuperative heat exchangers typically recover from 40 percent to 80 percent of the oxidation process energy. Most system designs fall into the range of 60 to 70 percent recovery. The successful use of metal recuperative heat exchangers is impacted by several factors. These factors include process exhaust temperatures, system operating temperature requirements, temperature stratification within the unit (which relates to flow turndown), the type and concentration of the VOCs treated, and the process operating cycle. These factors affect the efficiency and life of the unit. Temperature limitations of the metals in the heat exchangers along with stresses induced by changing process conditions can severely reduce equipment life.
Regenerative. Regenerative-type heat recovery systems typically use ceramic media to collect and store energy. The ceramic media is contained in multiple towers or canisters that are interconnected by ducting and a valve system. The valve system in regenerative designs directs the incoming exhaust stream between the various canisters of ceramic media. By switching from one tower of media to another — that is, cycling — one ceramic bed will release its energy while the ceramic bed in the other tower absorbs energy.
For both recuperative and regenerative heat recovery systems, the recovered energy is used to preheat the process exhaust as it enters the oxidation system.
Regenerative heat recovery systems are capable of recovering up to 97 percent of the energy used in the oxidation process. Most units operate in an energy recovery range of 85 percent to 95 percent. The ceramic media used in these systems is typically capable of continuous operating temperatures of 1800 to 1900°F (980 to 1040°C). High temperature capabilities — along with the use of hot-gas bypass systems — allow modern regenerative systems to operate effectively over a range of airflows with VOC concentrations of nearly 0 percent to 25 percent of the LFL (lower flammability limit for the VOC).
Catalytic. Catalytic oxidizers include the use of a catalyst to lower the operating temperatures required to destroy the VOCs. Typical catalyst formulations are either precious or base metals. The catalyst is deposited on a substrate, usually beads or monoliths, which is placed in the solvent-laden airstream. While catalysts lower operating temperatures and save energy, they are susceptible to poisons and masking agents that can reduce catalyst activity and VOC destruction capabilities.
System Design Criteria for Industrial Oxidizers
Airstream Composition. Once the capture system has been designed, the next consideration in defining the oxidation system is to quantify the concentration of VOCs and other contaminants in the exhaust stream. Using the design parameters for the oven system:
- The types of VOCs used in the process can be identified.
- The minimum and maximum solvent concentrations can be defined.
- The exhaust volumes and temperatures can be determined.
Any particulate that may be generated by the coating equipment or process chemistry also must be identified. These are all critical parameters used in the selection of the oxidation system.
Production Schedule. The operating schedule for the process is an important consideration. The variability in uptime of a process can have a tremendous impact on the operating efficiency and life expectancy of pollution control equipment. Catalytic oxidizers are ideal for processes that generate high solvent concentrations and are frequently online and offline. The lower operating temperatures can reduce the extreme stress on equipment components. Running schedules also impact oxidation system selection. Regenerative oxidizers are more efficient for low solvent concentrations, large airflows and continuous operation.
Particulates and Nonvolatile Oxidation Products. Upstream particulate matter along with downstream silicones, phosphates, clay, glass fibers, resin fragments and other inorganic substances can sometimes enter the exhaust stream. Even in concentrations as low as a few parts per million (ppm), these impurities can clog an oxidizer in a few months. Concentrations as low as 0.1 ppm can mask catalysts in less than a year.
Exhaust Rate and Temperature. Nothing is more important to the economic impact of an oxidizer than the exhaust rate and temperature. As the exhaust rate is reduced for a given process condition, the solvent concentration increases. Lower exhaust rates with higher solvent levels reduce the need for fuel in every type of oxidizer. However, once the LFL has reached 6 percent to 12 percent, fuel consumption is minimal for oxidizers with heat exchangers, so further exhaust rate reduction is not necessary.
Self-Sustaining Operation. The self-sustaining condition is the most desirable operating condition for any oxidation system. Only through careful optimization of process air requirements can this condition be achieved. The energy recovered in the oxidation process comes from the energy generated from burning of process solvents (an exothermic reaction) plus the energy introduced through an auxiliary burner (used in most systems). Increased energy available from the process solvents reduces the auxiliary fuel required to support the oxidation process. Regenerative thermal, regenerative catalytic and recuperative catalytic systems can be designed to operate using only the energy available from the process solvents. This operating condition is referred to as self-sustaining. In a self-sustaining mode, the oxidation system burners are shut down and energy from the process solvents entirely supports the oxidation process.
In some oxidizer applications, the self-sustaining condition is easily achieved, and excess energy exists from the process solvents. Once the oxidation process energy demand is satisfied, the excess energy could be used to reduce the energy demand of the process or other areas in the plant.
Secondary Energy Recovery Options. Secondary, and even tertiary, energy recovery systems are becoming more popular as a way to reduce energy costs. One example of uses for the excess energy would be operations where the energy is put directly back into the process by returning oxidizer exhaust to the oven. Energy recovery through the use of air-to-air heat exchangers or thermal fluid systems such as hot oil or water glycol exchangers is also an option. Some facilities have hot/chilled water or steam demands that can be supplied through the use of a waste heat boiler or adsorption chiller fired by the oxidation system exhaust. In northern climates, facilities are able to use this excess energy as makeup air to the building for heating purposes.
When looking at the potential for secondary and tertiary heat recovery systems, it is important to confirm that the process and pollution control system energy demands are met first. Only when the process and oxidation system energy demands are satisfied is it appropriate to investigate alternative energy recovery projects.
Achieving Optimal Process Design for Industrial Oxidizers
In summary, optimal process design can only be achieved through understanding multiple equipment variables and the interrelationships between drying and oxidation systems. Dryer design clearly affects emission control design and operation. Emission control design can have an effect on dryer operating costs and facility energy management. Optimal solutions balance the competing needs of all the variables and keep the total process in mind as each improvement is implemented.